An injection molding machine which melts feedstock composed of thermoplastic resin and injects it into a mold is configured to include a screw which is accommodated in the inside of an injection cylinder. Then, as the screw, a screw with a blade-shaped flight spirally provided on the peripheral surface of a shaft which is rotationally driven has been widely used. In such a screw, solid thermoplastic resin feedstock supplied to the inside of the injection cylinder is kneaded while being melted and plasticized by receiving a shear force from the screw which rotates. Then, after the melted thermoplastic resin is accumulated in the inside of the injection cylinder at a leading end portion of the screw, the screw advances in the axial direction, whereby the melted thermoplastic resin is injected into the mold. Here, in general, at the leading end portion of the screw, a check valve for thermoplastic resin, which is called a check ring, is provided so as to be movable in the axial direction.
Incidentally, in order to improve the mechanical characteristics of a molded product, addition of reinforcement fibers to the thermoplastic resin has been widely performed. Then, in a case where the thermoplastic resin containing the reinforcement fibers is injection-molded by the above-described screw, if excessive breakage occurs in the reinforcement fibers due to plasticization and agitation by the screw, a problem arises in the quality of a molded product. Therefore, a low shear type screw by which it is possible to reduce breakage of the reinforcement fibers has been proposed in the related art (refer to, for example, PTL 1). The screw of PTL 1 is characterized by the following three points. First, the thickness at a top portion of a flight is formed to be a size in a range of 0.2 times to 0.4 times the diameter of the screw. Second, a step of less than or equal to 1 mm toward the radial direction of the screw is formed at the top portion of the flight on the leading end side of the screw. Third, the movement stroke of a check ring is set to be in a range of 5 mm to 20 mm.
However, in the screw of PTL 1, although breakage of the reinforcement fibers can be reduced, since the screw is a low shear type, compared to a case of using a general screw for resin molding, a problem of so-called insufficient dispersion arises such that disaggregation and dispersion of the reinforcement fibers in the thermoplastic resin is inferior. Therefore, as means for preventing the problem of insufficient dispersion, a so-called double flight screw provided with two flights has been used in recent years (refer to, for example, PTL 2). This double flight screw is constituted by providing a main flight over the entire axial length of the screw and also providing a sub-flight having a height lower than that of the main flight between adjacent main flights. With such a configuration, since thermoplastic resin is also agitated by the sub-flight in addition to the main flight, dispersion of the reinforcement fibers in the thermoplastic resin is promoted.
However, in the double flight screw disclosed in PTL 2, the thickness of the gap between a tip end of the sub-flight and the inner wall surface of an injection cylinder is formed to be a constant size. Therefore, if the gap thickness is set small, immediately after the thermoplastic resin is input from a feedstock input port (a hopper), softening of the thermoplastic resin is insufficient at a rear end portion of the screw. Therefore, the gap between a tip end in the radial direction of the sub-flight and the inner wall surface of the injection cylinder is clogged with the thermoplastic resin, whereby plasticization defects (resin transportation defects) occur. Further, at the rear end portion of the screw where softening of the thermoplastic resin is insufficient, the thermoplastic resin cannot absorb the shear force that it receives from the screw through bending deformation. Therefore, if the thermoplastic resin is subjected to a large shear force between the tip end in the radial direction of the sub-flight and the inner wall surface of the injection cylinder, the shear force is also propagated to the reinforcement fibers as is, and thus excessive breakage occurs in reinforcement fibers. In addition, in the present invention, the hopper side that is upstream side in the resin transportation by the screw is expressed as a screw rear-end portion (or a screw rear-end side) and the downstream side is expressed as a screw leading-end portion (or a screw leading-end side).
On the other hand, if the gap thickness is set large, breakage of the reinforcement fibers at the screw rear-end portion can be suppressed. However, since a shear force and a stretching force which are loaded to the thermoplastic resin sufficiently softened due to a rise of temperature at the screw leading-end portion between the tip end in the radial direction of the sub-flight and the inner wall surface of the injection cylinder are absorbed by deformation of the softened thermoplastic resin, the shear force and the stretching force cannot be effectively loaded to the reinforcement fibers which are suspended in the inside of the thermoplastic resin. Therefore, disaggregation defects or dispersion defects occurs in the reinforcement fibers contained in the thermoplastic resin.
Incidentally, as other means for preventing the problem of insufficient dispersion that a screw generally has, a method that a so-called mixer is provided in plural stages in the axial direction at a leading end portion of the screw has been proposed in the related art (refer to, for example, PTL 3). In the mixer, a plurality of fins protruding in the radial direction is provided at regular intervals in the circumferential direction.